1. Field of the Invention
The present invention relates to an apparatus and a method for forming a thin film of zinc oxide on a base member by electrodeposition, and more particularly to a thin zinc oxide film-forming apparatus and method in which a distance between a counter electrode and a base member is adjusted by improving the shape of the counter electrode, and a thin zinc oxide film forming apparatus and method in which replenishment of zinc ions is improved.
2. Related Background Art
The photovoltaic element composed of hydrogenated amorphous silicon, hydrogenated amorphous silicon germanium, hydrogenated amorphous silicon carbide or microcrystalline silicon has conventionally been provided with a back surface reflection layer in order to improve the light collecting efficiency in the long wavelength region. It is preferable that such back surface reflection layer shows effective reflection characteristics in a wavelength region close to the end of the band of a semiconductor material where its absorption becomes lower, namely in a range from 800 nm to 1200 nm. Such condition can be sufficiently satisfied by a metal layer such as of gold, silver, copper or aluminum.
There is also known so-called light confinement technology by providing a layer with surface irregularities which is optically transparent within a predetermined wavelength region. Such layer with surface irregularities is generally provided between the above-mentioned metal layer and a semiconductor layer to improve the short-circuit current density Jsc by effective utilization of the reflection layer. Also for preventing the deterioration of the characteristics by a shunt path, it is known to provide a conductive layer of a light-transmissive material, namely a transparent conductive layer, between the metal layer and the semiconductor layer. Broadly speaking, these layers are deposited by vacuum evaporation or sputtering, and provide an improvement of 1 mA/cm.sup.2 or more in terms of the short-circuit current density Jsc.
As examples of these configurations, the reflectivity of the reflection layer consisting of silver atoms and the textured structure are investigated in the Extended Abstract of the 51st Autumn Meeting of the Japan Society of Applied Physics (1990), 29p-MF-2, p747 "Optical confinement effect in a-SiGe solar cells on stainless steel substrates" and Sannomiya et al., Technical Digest of the International PVSEC-5, Kyoto, Japan, p381, 1990 "P-IA-15a-SiC/a-Si/a-SiGe multi-bandgap stacked solar cells with bandgap profiling". In these examples, effective irregularities are formed by depositing silver with change in substrate temperature to obtain two reflection layers, and an increase in the short-circuit current is achieved by the optical confinement effect and the combination with a zinc oxide layer.
Conventionally, a zinc oxide layer is formed by sputtering, ion plating or CVD. The zinc oxide layer adapted for use in the photovoltaic element can be formed by sputtering, and desirable results can be easily obtained particularly by magnetron sputtering.
A zinc oxide layer consisting of hexagonal polycrystals can be formed on a substrate opposed to a target, by employing sintered zinc oxide as the target, generating a surface magnetic flux density of 50 to 800 Gauss on the surface of the target and applying a voltage. A sputtering gas can be composed of argon eventually added with oxygen, and the sputtering pressure is 1 Torr or less.
However, the zinc oxide film prepared in this method shows an insufficient light confinement effect in the wavelength range of 600 nm to 1000 nm, and there are drawbacks of a high preparation cost of the target material and a high depreciation cost of the vacuum apparatus.
As alternative methods, there are also known wet methods such as the spray pyrolysis method or sol-gel method, but the necessity of heating the substrate to about 300.degree. C. to 800.degree. C. limits the substrates to be used, thereby significantly increasing the cost of manufacturing the photovoltaic element by these methods, thus posing a major barrier for industrial application of the solar cells.
As a countermeasure for the above problems, there has been reported electrochemical deposition of a thin transparent zinc oxide film by supplying a current from a counter electrode immersed in an aqueous solution of zinc nitrate, as disclosed in the specification of the Japanese Patent Application No. 7-23775, which corresponds to Japanese Patent Application Laid-Open No. 8-217443, and Masanobu Izaki and Takahasi Omi, "Electrolyte optimization for cathodic growth of zinc oxide films", Journal of Electrochemical Soc., Vol. 143, No. 3. These methods dispensing with an expensive vacuum apparatus or an expensive target can drastically reduce the manufacturing cost of the zinc oxide film. Also these methods are effective for the photovoltaic element with a large area such as solar cells, because the deposition can be also conducted on a large-area base member.